CNMI and Guam stormwater management criteria

Horsley Witten Group

Guam and the Commonwealth of the Northern Marina Islands (CNMI) receive a lot of rain! The average annual rainfall exceeds 100 inches per year in many locations. These climatic conditions combined with the region’s unique limestone, volcanic geologic formations, sensitive water resources and significant land development forces make stormwater a very significant environmental and economic issue. Historically, stormwater has been viewed as strictly a drainage issue and has been routed to the nearest discharge location, has been infiltrated into the highly-permeable limestones with little or no pre-treatment, or has been conveyed directly to receiving waters. Along with development comes increased amounts of impervious surfaces, precluding the natural infiltration of rainwater into the underlying groundwater system. As a result, the groundwater “lens” (which serves as the principle drinking water source) is depleted. Or, in the instances where stormwater is infiltrated without adequate pre-treatment, groundwater quality is degraded. This report presents an analysis of these important issues and presents a recommended approach to improve upon current stormwater management practices. Management strategies and technologies, which have been implemented in other areas, have been reviewed and where appropriate have been incorporated into recommendations for consideration in Guam and CNMI.